There is demand for both a wide band and favorable temperature characteristics to be satisfied in a duplexer and an RF filter used in a mobile communication system. Conventionally, a piezoelectric substrate configured from 36° to 50° rotated Y-cut X-propagation lithium tantalate (LiTaO3) has been used in a surface acoustic wave apparatus used in a duplexer or an RF filter. The TCF (Temperature coefficient of frequency) of the piezoelectric substrate has been approximately −40 to −30 ppm/° C. Also, in order to improve the temperature characteristics, there is known a method of forming a silicon oxide (SiO2) film having a positive TCF so as to cover the IDT electrodes on the piezoelectric substrate.
On the other hand, with an object other than improving the TCF, Patent Document 1 (Japanese Laid-open Patent Publication No. 11-186866) discloses a manufacturing method for a surface acoustic wave apparatus in which an insulating or semiconductive protective film is formed so as to cover the IDT electrodes of the surface acoustic wave apparatus.
Also, Patent Document 2 (Japanese Laid-open Patent Publication No. 61-136312) discloses a 1 port surface acoustic wave resonator configured by forming an electrode made of a metal such as aluminum or gold on a piezoelectric substrate made of crystal or lithium niobate (LiNbO3), further forming an SiO2 film, and thereafter planarizing the SiO2 film. Planarizing the SiO2 film in this way obtains favorable resonance characteristics.
Also, Patent Document 3 (Japanese Patent No. 3885824) discloses a configuration including a piezoelectric substrate configured from LiNbO3 having an electrical mechanical coupling coefficient (k2) of 0.025 or more; at least one electrode that is formed on the piezoelectric substrate and is made of a metal whose density is greater than that of Al, an alloy whose main component is the metal, or a laminated film configured from either a metal whose density is greater than that of Al or an alloy whose main component is the metal, and another metal; a first insulating layer formed in a region other than a region where the at least one electrode is formed, such that a film thickness of the first insulating layer is approximately equal to that of the electrode; and a second insulating layer formed so as to cover the electrode and the first insulating layer, wherein the density of the electrode is 1.5 or more times that of the first insulating layer, the thickness of the second insulating layer is in the range of 0.18λ to 0.34λ (where λ is the wavelength of the surface waves), and the projection height of a convex portion on the surface of the second insulating layer is 0.03λ or less (where λ is the wavelength of the surface waves). With the configuration disclosed in Patent Document 3, the reflection coefficient of the IDT electrodes is sufficiently large, and the deterioration of characteristics due to ripples appearing in resonance characteristics and the like does not readily occur.
However, the configurations disclosed in the patent documents have the disadvantage that, as illustrated in FIG. 15, unnecessary waves B appear at a higher frequency than the main response A that is the object in the frequency response of the absolute value of the admittance (S coefficient) of the acoustic wave device. The characteristics illustrated in FIG. 15 are the result of performing a simulation with an FEM (Finite Element Method) using the physical properties illustrated in Table 1 on an acoustic wave device 200 that, as illustrated in FIG. 16, includes IDT electrodes 202 whose period λ is 2 μm on a piezoelectric substrate 203, and furthermore includes an SiO2 film 201 covering the IDT electrodes 202.
TABLE 1Physical properties that were usedYoung'sPoisson'sAcousticSubstancemodulusratioDensityVelocityimpedance(unit)(GPa)(—)(kg/m3)(m/sec)(Ns/m3)SiO270.70.252300554412.8Au78.50.4219260201938.9SiC2890.182920994829.0
As illustrated in FIG. 15, if an unnecessary response (unnecessary waves B) exists, there is the problem that suppression outside the passband degrades when a filter is formed using the acoustic wave device.
The relationship between unnecessary wave size and the film thickness of the SiO2 film is illustrated in FIG. 17, and although unnecessary waves can be suppressed by reducing the film thickness of the SiO2 film, there is the problem that, as illustrated in FIG. 18, the temperature characteristics deteriorate if the film thickness of the SiO2 film is reduced. Note that FIG. 18 illustrates the relationship between the film thickness of the SiO2 film and TCF.